High temperature allotropy and thermal expansion of the rare-earth metals

Abstract

By means of high temperature X-ray techniques the crystal structure of lanthanum, cerium, praseodymium, neodymium, ytterbium, and possibly gadolinium was found to be body-centered cubic at temperatures near their respective melting points. For ytterbium a hexagonal close-packed structure was also observed, which was shown to be stabilized by atmospheric impurities. Evidence for possible high temperature crystalline transformations in gadolinium, terbium, dysprosium, holmium, and lutetium was obtained by means of electrical resistance measurements; erbium gave no such evidence. X-ray data were used to derive empirical equations which describe thermal expansion coefficients of scandium, yttrium and the rare-earth metals. Europium exhibits a rapidly decreasing coefficient of expansion with increasing temperature, which may be a consequence of a gradual promotion of one of the 4f electrons into the conduction band. The hexagonal rare-earth metals were found to have nearly the same axial ratio at their respective transformation temperatures.